1. Field of the Invention
The present invention relates to a differential amplifier used for amplifying a read signal in a magnetic memory device, i.e., a hard disk, a semiconductor memory device, and the like.
1. Description of the Related Art
A differential amplifier for amplifying a read signal in a hard disk or a semiconductor memory device has to satisfy the following two requirements:
1) a differential input capacity is as small as possible; and PA1 2) a noise level in an output signal is as small as possible.
For example, in a hard disk having an increased memory density, the frequency of a read signal is higher. Therefore, when the differential input capacity of the differential amplifier as a read amplifier is large, it is difficult to sense the read signal. Also, when the amplitude of the read signal is small, i.e., the S/N ratio thereof is small, a high noise level in the differential amplifier may invite a read error.
A first prior art differential amplifier includes an input stage for receiving and amplifying input signals, and an output stage formed by emitter followers for receiving amplified signals from the input stage to generate output signals. This will be explained later in detail. In the first prior art differential amplifier, however, the differential input capacity is still large, although the noise level of the output signals is small.
In a second prior art differential amplifier, the output signals of the output stage are fed back to the input stage (see: JP-A-HEI2-223209). This will be explained later in detail. In the second prior art differential amplifier, however, the noise level of the output signals is still large, although the differential input capacity is small.